Thiocarlide (THC), a thiourea, inhibits the synthesis of mycolic acids and fatty acids in Mycobacterium tuberculosis (M. tb). Our evidence indicates a unique mode of action of THC on fatty acid synthesis, namely, the inhibition of the synthesis of oleic acid attributable to the inhibitory effect of THC on the activity of the membrane-associated stearoyl-CoA ( 9) desaturase DesA3. Although THC has at least one other enzymatic target in the mycolic acid biosynthetic pathway, this(ese) enzyme(s) remain(s) to be identified. Since our preliminary data indicate that THC has no inhibitory effect on mycolic acid synthesis in Corynebacterium and Nocardia species and on the activity of the FAS-I and FAS-II systems from Mycobacterium aurum, the second target of the drug is likely to differ from that of other anti-TB drugs such as isoniazid and ethionamide and could be an enzyme introducing early functional groups into the meromycolate chain of mycolic acids. The genome of M. tb potentially encodes seven proteins related to epoxide hydrolases, some of which appear to be involved in the introduction of functional groups in mycolic acids and to be sensitive to THC and other more potent urea/thiourea inhibitors. These enzymes will be purified and their inhibition by THC directly tested in vitro. Towards a broader genetic approach, we have isolated a series of THC- and urea/THC (cross-)resistant mutants of M. tb and undertaken to characterize the mutations responsible for conferring high level of resistance to these drugs. The essentiality and function of the newly identified target(s) of THC will be established. The three-dimensional structure of the most relevant targets of the drug will be determined and cell-free assays developed to allow screening of inhibitors, including our existing libraries of urea and thioureas derivatives, and facilitate at a later stage a medicinal chemistry approach in the form of chemical synthesis and extended testing of products. THC was once acceptable for treatment of tuberculosis but lost favor due to untoward absorption kinetics and low bioavailability. This approach is designed to overcome these aspects. From a more fundamental point of view, the elucidation of the mode of action of THC also represents a unique opportunity to identify the as yet unknown enzymes responsible for the introduction of double bonds and oxygenated functions in the meromycolate chain of mycolic acids. PUBLIC HEALTH RELEVANCE: In the context of the threat of MDR-TB and XDR-TB, new drugs against tuberculosis are urgently needed. We propose to elucidate the mode of action of thiocarlide, a drug that was part of the clinical treatment of tuberculosis in the 1960s, and to exploit this knowledge to develop novel therapeutic agents with the same mode of action but much improved pharmacological properties.